As critical dimensions of semiconductor devices continue to scale smaller, control over device variation is increasingly important. The geometric variation of fins translates into varied electrical performance across a wafer. Methods to control the line edge roughness (LER) at the photolithography level, especially with EUV resist on smaller feature sizes, is an increasingly high value issue.
Conventional LER reduction of 248 nm styrene-based and 193 nm acrylate-based photoresist may be accomplished using plasma treatment, ion bombardment, and thermal treatments. However, manipulation of the chemical structure of the resist through various plasma chemistries and direct physical sputtering has diminishing returns as the feature size drops below 20 nm. Not only is the photoresist volume far smaller, making modification of the resist difficult without removing a substantial percentage of the material, but the decreased surface area to volume ratio inhibits reflow of the polymer.
Furthermore, the significantly smaller resist volume of such features makes reflow and smoothing of the resist lines more difficult using conventional ion implant processes. Moreover, EUV lithography used for exposing many of the sub-20 nm features suffers from severe shot noise, often resulting in pinched resist lines and high LER. When treating the pinched lines via ion implant, failure of the resist lines as a result of the ion sputtering is common.